Because this 'feature' that you cannot disable (without a hack or creating your own page type) literally parses the page and adds a snippet of the content to the email bypassing all security in place.  I understand that as a user they can simply copy and paste the contents a of page and email them, but that requires effort and a strong understanding of what they are doing as well as giving the end user a chance to filter and/or remove content.   

What if your sharing the page with an external user?  Now they can just click the button, enter an email address... any email address, and send content from your page to anyone.   Its pretty amazing that this hasn't been addressed yet.